Method of providing a protective coating on magnesium and its alloys



Patented June 22, 1943 METHOD OF PROVIDING A PROTECTIVE COATING NMAGNESIUM AND ITS AL- LOYS William S. Loose and Herbert K. De Long,Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Michigan No Drawing. Application November 14, 1938,

15 Claims.

The invention relates to methods of producing protective coatings onarticles of magnesium and magnesium base alloys. It more particularlyconcerns an improved method of producing on articlcs of the aforesaidmetals a coating which is highly resistant to corrosion and adapted tobe used as an improved base for applying paints, varnishes, lacquers,and the like.

Various methods have been proposed heretofore for producing protectivecoatings on magnesium and its alloys by subjecting articles thereof tothe action of chemical solutions, among which the aqueous solutions ofcertain of the oxygenated inorganic compounds which produce adherentcoatings are the most widely used, such as aqueous solutions containingchromates or bichromates of the alkali metals.

We have now found that new and greatly improved results are obtained if,instead of subjecting the article of magnesium or an alloy thereofdirectly to the action of an aqueous solu- Serial No. 240,345

fer to use those having an acid reaction as Is obtained by dissolvinghydrofluoric acid or an acid fluoride in water in a concentration ofabout 2 per cent or more by weight. A generally useful fluoride solutionis a 25 to 30 per cent solution of hydrofluoric acid. Otherconcentrations may be used, if desired.

In carrying out this step of the process, the article is brought intocontact with preferably an aqueous solution of the soluble fluoride forfrom about 1 to 60 minutes or more at ordinary temperatures. If desired,however, the temperature may be as high as 80 C. or even higher, wherebytion of an oxygenated inorganic compound, the

which is capable of forming water-soluble salts of magnesium, such asnitric or sulphuric acid.

The cleaned article is then, according to the invention, subjected tothe action of a solution containing a soluble fluoride by spraying thesolution on the metal or by immersing the metal in the solution or inany other manner in which the solution and the article may be broughtinto contact for a suflicient length of time to bring about theformation of a coating. When the fluoride is used in aqueous solution,the coating is so thin as to be diflicult to analyze in substance withcertainty. It is believed, however, to be largely magnesium fluoride.Molten fluoride baths definitely yield a coating of magnesium fluoride.Various soluble fluorides may be used to make a solution or bathsuitable for producing this coating. For example, there may be usedwater soluble fluorides such as hydrofluoric acid, the acid fluorides orbi-fluorides such as potassium bi-fluoride, ammonium bi-fluoride and thelike, the neutral alkali metal fluorides such as sodium and potassiumfluoride and combinations of these, and some of the more complexfluorides may be used such as fluosilicic acid (HzSiFs) andhydrofluoboric acid (HBF4) Of these fluorides we prethe action of thesolution will be expedited, provided excessive volatilization of HFvfrom the solution does not occur. If the soluble fluoride bath containsmainly hydrogen fluoride, it is best to use the bath at about roomtemperature so as to avoid incurringa loss of HF, which is fairly easilyvolatilized from the solution at higher temperatures. The action of thesolution also may be expedited, if desired, by applying apotential tothe article,- this being made the anode. A suitable voltage to employ isfrom 90 to 100 ormore. At ordinary temperatures anodizing for from about5 to 15 minutes in the fluoride bath as described will produce asatisfactory coating. Baths comprising a molten fluoride also may beused for depositing a fluoride coating, the article being made theanode, and electrolyzed to effect the deposition. Fused potassium orsodium fluoride, bifluoride, and mixtures of dehydrated boric acid withpotassium bifluoride may be used at about 200 C. Voltages up to 220 maybe used.

After having thu subjected the article to the action of the fluoridesolution or bath, as described, it is preferably rinsed in water. Thearticle is then ready for the next step. In this it is subjected, as byimmersion, to the action of an aqueous solution of a salt of an oxy-acidof an element selected from the group consisting of chromium,molybdenum, phosphorus, selenium, titanium, tungsten, vanadium,especially the alkali metal and ammonium salts of such oxypound beingreduced by the action of the metal to form an insoluble magnesiumcompound. The following are typical examples of such salts: sodiumchromate and bi-chromate, ammonium molybdate, the sodium phosphates,ammonium selenate, sodium titanate, sodium tungstate, and sodiumvanadate.

The oxygenated compound may be employed in aqueous solutions havingvarious pH values, best results being obtained when the pH value ofabout 10.2 is avoided, which is the pH value corresponding to theformation of magnesium hydroxide by precipitation. For example, we havefound that the pH value of the bath may lie advantageously eitherbetween about 0.5 and 8 or between about 12 and 14. The concentration inwhich to employ the oxygenated compound varies more particularly withthe pH at which the solution is to be used and may also be limited bythe solubility of the particular oxygenated compound employed. We havefound that in the lower range of pH values a considerable concentrationof the oxygenated compound may be employed as, for example, from aboutto 40 per cent by weight, while at the higher range of pH values only arelatively low concentration is desirable, such as from about 0.1 to 4per cent of oxygenated compound, although in some instances as much as10 per cent may be used. The pH value of the solution may be lowered orraised, if desired or necessary, by adding either an acid such assulphuric acid, or an alkali, preferably of the alkali metals, such assodium hydroxide,

respectively, thus regulating the pH of the solution of the oxygenatedcompound to a value between about 0.5 and 8 or between about 12 and 14,as desired. In the foregoing ranges of pH values, we prefer to use from4 to 6 in the lower range and from 13 to 14 in the higher range.

The duration of the treatment in the solution containing the oxygenatedcompound varies with its temperature, concentration, and pH value. Ingeneral, a treatment of from 5 to 60 minutes and preferably from tominutes at or near the boiling temperature of the solution producessatisfactory results. Similar results can be obtained at temperatures upto 70 C. and in from 5 minutes to one hour by making the article theanode and electrolyzing in a solution having a pH between 0.5 and 8,with a current density of from 5 to amperes of anodizing current persquare foot of surface. When the pH of the solution is between 12 and14, superior results are obtained, particularly if the voltage employedis above 15. If desired, an alkaline solution can be used for theanodization, as by adding about 2 per cent of sodium hydroxide to thesolution containing the oxygenated compound. Suitable concentrations inwhich to employ the oxygenated compound for best results when thearticle is to be anodized vary with the specific compound used asfollows: with chromates, the concentration may be about 0.2 per cent;with molybdates, about 5 to 6 per cent; phosphates, about 3 per 'cent;selenate, about 1 per cent; titanate, about 1 per cent; tungstate, about3 per cent; and vanadate, about 5 per cent, although otherconcentrations maybe used.

After the article has been subjected to the action of the solution ofthe oxygenated inorganic compound either with or without anodizing, as

described, following the treatment with a fluoride, the article may berinsed in water and dried.

The following examples are illustrative of modes of carrying out theinvention.

Example 1 A casting of a magnesium alloy consisting of 6 per cent ofaluminum, 3 per cent of zinc, 0.1 per cent of manganese, the balancebeing magnesium, was sand blasted, then cleaned by a 10 second clip in awater solution containing 8 per cent of nitric acid and 2 per cent ofsulphuric acid, as is the usual practice of preparation for a chemicalfinish. The cleaned casting was then immersed for 5 minutes in a 15 percent solution ,of hydrofluoric acid at room temperature, followed by acold water rinse. The so treated cast- 7 ing was then immersed for 45minutes in a boiling 10 per cent solution of sodium dichromate(Na-2Cr'20'z-2H2O) rinsed in cold water, and dried.

Example 2 sodium bichromate and 2 per cent of sodium hydroxide for 10minutes at 50 C. while being anodically electrolyzed with a potential of45 volts, the pH of the bath being 13.4. The casting was then removedfrom the solution, rinsed and dried. The ability to resist corrosion wasthen determined by subjecting the article to the usual alternateimmersion test in a 3 per cent solution of salt at F. for one week andmeasuring the area of surface attacked. Castings treated as abovedescribed showed only 3 to 6 per cent as much area corroded as similarcastings treated in either the above solution of the oxygenated compoundwithout initially treating the article in the fluoride solution or theabove fluoride solution alone.

. Example 3 An article of a magnesium base alloy consisting of 4 percent of aluminum, 0.3 per cent manganese, the balance being magnesium,in the extruded condition, was immersed for 12 minutes in a 25 per centsolution of hydrofluoric acid at' 25 C. After this treatment, thearticle was transferred to a boiling 10 per cent solution of sodiummetavanadate for 10 minutes, then rinsed and dried. The resistance tocorrosion of the so treated article was found tube from 5 to 7 times asgreat as the resistance to corrosion obtained upon treating similararticles in either of the two above described solutions.

Example 4 cent solution of hydrofluoric acid for 15 minutes at 28 C. andthen to the action of a boiling aqueous solution containing 12 per centeach of sodium bichromate and sodium dihydrogen phosphate. Theresistance to corrosion was found to b more than twice as great assimilar articles treated in either of the two above solutions.

The coating produced according to the method provides an excellent basefor the application of paint, varnish, or the like-in that the varnishis made to adhere better and in combination with the chemical coatinggenerally provides additional protection against corrosive attack.

Other modes of applying the principle of the invention may be employedinstead of those explained, change being made as regards the methodherein disclosed, provided the steps stated by any of the followingclaims or the equivalent of such stated steps be employed.

We therefore particularly point out and distinctly claim as ourinvention:

ing upon articles of magnesium and magnesium base alloys, the stepswhich consist in subjecting the article to the action of a bath, theactive constituent of which-is a fluoride, to produce a coating on thearticle, and then subjecting the coated article to the action of anaqueous solution of a water-soluble salt of an oxy-acid of an elementselected from the group consisting of chromium, molybdenum, phosphorus,selenium, titanium, tungsten, and vanadium to increase the corrosionresistance of the coating thereon.

2. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys, the steps which consist insubjecting the article to the action of an aqueous solution containing asoluble fluoride as active constituent, to produce a coating upon thearticle, and then subjecting the coated article to the action or anaqueous solution of a water-soluble salt selected from the classconsisting of the alkali metal and base alloys, the steps which consistin subjectin the article'to theaction of an aqueous solution containinghydrofluoric acid to produce a coating on the article, and thensubjecting the coated article to the action of an aqueous solutioncontaining sodium chromate andhaving a\pH value between 12 and 14toinciease the coi'rbsion resistance oi the coating thereon.

ammonium salts of the oxy-acid of the elements chromium, molybdenum,phosphorus, selenium, titanium, tungsten, and vanadium to increase thecorrosion resistance of the coating thereon.

3. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys, the steps which consist insubjecting the article to the action of an aqueous solution containing asoluble fluoride as active constituent,

to produce a coating upon the article, and then subjecting the coatedarticle to the action of an aqueous solution of a soluble salt ofchromic acid to increase the corrosion resistance of the coatingthereon.

4. A method according to claim 3 wherein the soluble chromate is analkali metal chromate.

5. A method'according to claim 2 wherein the oxy-acid salt is sodiumvanadate.

6. A method according to claim oiw-ac d salt is a phosphate.

2 wherein the '7. A method according to claim 2 wherein the fluoride isan acid fluoride.

8. A method according to claim 3 wherein the fluoride is hydrofluoricacid and the soluble chromate is an alkali metal chromate.

9. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys, the steps which consist insubjecting the article to the action of an aqueous solution containinghydrofluoric acid to produce a coating on the article, and thensubjecting the coated article to the action otan aqueous solutioncontaining sodium bi-chromate and having a pH value between 0.5 and 8 toincrease the corrosion resistance of the coating thereon.

11. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys, the,steps which consist insubjectin the article to the action of an aqueous solution containing asoluble fluoride while. anodically electrolyzing the same to produce acoating on the article, and thereafter subjecting the coated article tothe action of an aqueous solution containing an alkali metal chromate toincrease the corrosion'resistance' oi the coating thereon.

12. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys, the steps which consist insubjecting the article to the action of an aqueous solution containing asoluble fluoride to produce a coating on the article, and thereaftersubjecting the coated article to the action of an aqueous solutioncontaining an alkali metal chromate while anodically electrolyzing thesame to increase the corrosion resistance of the coating thereon.

13. In a method of producing a protective coating upon articles ofmagnesium and magnesium base alloys,the steps which consist insubjecting th article to the action of an aqueous solution containing asoluble fluoride while anodically electrolyzing' the same to produce acoating on the article, and thereafter subjecting the coated article tothe action of an aqueous solution containing an alkali'metal chromatewhile anodically electroiyzing the same to increase th corrosionresistance of the-coating thereon.

14. In a method oi producing a protective coating upon articles ofmagnesium and magnesium base alloys, the steps which consist insubjecting the article to the action or a solution containinghydrofluoric acid at room temperature to produce a'coating on thearticle, and thereafter subjecti-ng the coated article to the action ofan 10. In a method of producing a protective coataqueous solutioncontaining sodium bi-chromate at the boiling temperature to increase thecorrosion resistance of the coating thereon.

15. In a method or improving the corrosion resistance of an article ormagnesium and magnesium base alloys, said article having a magnesiumfluoride coating thereon, the step which consists in subjecting thearticle to the action oi an aqueous solution or a soluble alkali metalchromate.

WIILIAM- S. LOOSE. HERBERT K DE LONG.

